Mos activates MAP kinase in mouse oocytes through two opposite pathways

Marie-Hélène Verlhac^1,2, Christophe Lefebvre¹, Jacek Z. Kubiak^1,3, Muriel Umbhauer¹, Pascale Rassinier¹, William Colledge⁴, and Bernard Maro¹

¹Biologie Cellulaire et Moléculaire du Développement, UMR 7622, CNRS/Université Pierre et Marie Curie, 9 quai Saint Bernard–Bat. C-5, 75252 Paris, cedex 05, France and ⁴Wellcome/CRC Institute of Cancer and Developmental Biology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QR, UK ³Present address: UPR41 CNRS/Université de Rennes I, Rennes, France ²Corresponding author e-mail: Marie-Helene.Verlhac{at}snv.jussieu.fr

Abstract: Activation of mitogen-activated protein kinase (MAPK) in maturing mouse oocytes occurs after synthesis of Mos, a MAPKKK. To investigate whether Mos acts only through MEK1, we microinjected constitutively active forms of MEK1 (MEK1S218D/S222D referred herein as MEK*) and Raf (Raf) into mouse oocytes. In mos^–/– oocytes, which do not activate MAPK during meiosis and do not arrest in metaphase II, MEK* and Raf did not rescue MAPK activation and metaphase II arrest, whereas Mos induced a complete rescue. MEK* and Raf induced cleavage arrest of two-cell blastomeres. They induced MAPK activation when protein phosphatases were inhibited by okadaic acid, suggesting that Mos may inhibit protein phosphatases. Finally, in mos^–/– oocytes, MEK* induced the phosphorylation of Xp42^mapkD324N, a mutant less sensitive to dephosphorylation, showing that a MAPK phosphatase activity is present in mouse oocytes. We demonstrate that active MAPKK or MAPKKK cannot substitute for Mos to activate MAPK in mouse oocytes. We also show that a phosphatase activity inactivates MAPK, and that Mos can overcome this inhibitory activity. Thus Mos activates MAPK through two opposite pathways: activation of MEK1 and inhibition of a phosphatase.

Key Words: Keywords: meiosis/MEK1/mouse oocyte/phosphatase/Raf-1